This grant proposal presents continuing experiments on the molecular details of the receptor for tetrodotoxin (TTX) and saxitoxin (STX) at the sodium channel and on the measurement of various physiological and pharmacological activities to time the appearance of specific aspects of sodium channel function during the growth and differentiation of excitable cells. Molecular properties of the receptor will be characterized by competition studies with various metal and heteronuclear organic cations and by comparing the kinetic properties of saxitoxin and several STX derivatives of known structure. Growing and differentiating nerves and muscles will be assayed for drug-induced ion fluxes, electrophysiological properties, and saxitoxin binding.